Method and apparatus for controlling fractionating columns



June 3,

'c. J. SCHILLING METHOD AND APPARATUS FOR CONTROLLING FRACTIONATINGCOLUMNS Filed May 25, 1949 I N VEN TOR.

CLARENCE J. SCHILLING suchusefulness by high Patented June 3, 1952METHOD AND APPARATUS Foe ooN'rRoL- LING FRACTIONATING COLUMNS ClarenceJ. Schilling, Allentown, Pa., assignor to Air, Products, Incorporated.a. corporation of Michigan; I

" Application May 23, 1949jS erialNo.9437? 7 Glaims.

This invention relates to automatic controls for fra'ctionating: columns='andmore particularly to a method-of and apparatus for the automaticcontrol of the purity of -a'product a iracti'onatingcolumn. I

produced in well known that the-- purity of-"a' product from afractionating column'ma-y be controlled by varyingthe rate at which theproduct is withdrawn; '-'On-=increasing'- the rate of withdrawal,

the purity diminishes; on-decreasing the withdrawal-ra'te; the purityrises. This a commonly used "step fo'rcontrolling product purity. -.The

adjustment; of the product outlet valveis made manually by :the operatorguided by observation 'overthe outletvalve:opening which-would beresponsive directly to purity,- asf'or example by the use of athermal-conductivity purity meter; :however; such' instruments arereduced in practical cost' and great delicacy of construction;

' This invention'has as an object the provision of a method-andapparatus for the automatic control of the purity of the bottom productproduced in fractionating a mixture; bycontrolling the rate "at whichthe bottom product is withdrawn.

A further object is to control automatically the withdrawal of thebottom product by taking advantage or the wide changes in thetemperature differential between difierent-levels in fractionating a mixture.

"Another object of the invention is to provide in thefractionation of amixture control of the piirity of the product at the bottom of thecolumn by utilizing changes in the temperature differential between thebottom of the column and another point in the column at a higher level.

Ifhese and other objects are accomplished by the present inventionwherein one means responsive to changes intemperature is provided at thebottom of a fractionating column where one component is atthema'ximum'purity, and another suchmeans'is provided in theneighborhood of a level higher inthe column where the concentration of asecond component is beginning to drop from a maximum and theconcentration of a third and more volatile component is negligible butis beginning'to increase. so that changes in the temperaturedifferential between the two levels are utilized-to control' the rate ofwithdrawal of the first component and thus stabilize the purity of thiscomponent.

' While' this invention is applicable to the-control of the purity of aproduct obtained in the fractionation of any normally gaseous mixture,

it is found especiallyuseiul in connection with the' fractionation ofair, wherein the -purity of volume, thusdisregardi-ng the minutequantities of the rarer elements which do not affect materiallythecomposition of the products obtained in the separation of the air "intooxygen and nitrogen. I

Usually noattentionis given to the. argon content of theair.--'Argo'n"has a boiling point'about 3 C. below 'that of oxygen andabout 10- C. above that-of nitrogen. Consequently, in the normalrectification of-' air, the argon tends to follow the'oxygen to thelower levels of the rectii'lertand thus tends to contaminate the oxygenproduct.

The concentration of the argon in both the liquid and Vaporsincreasesmaterially until a point at a certain levelin the rectification columnis reached'where the: argon concentrationis at a maximum. In practice,the percentage of argon accumulating at this point in the column isbetween 'Ia'n'd 12%. 1 v.

Since the boiling 'anddew-curves of argonoxygen mixtures lie closetogether,uthe mixture is. more difllcult to "separate than anoxygennitrogen mixture. Thus, on the lower-plates of a fractionatingcolumn for a tertiary mixture such as air, the separation is between theargon and the oxygen, and only a'negligible amount of nitrogen ispresent. At a level about one third of the way up from the bottom of thecolumn, the percentage of argon on each plate begins to decrease fromthe maximum concentration, and the percentage of the nitrogen starts toincrease. It is this level in the column that is utilized in the presentinvention for the location of the upper control means. 3 Y

At constant pressure, the boiling point of liquid oxygen-rich mixtureriseswith increasing approach to purity in oxygen. If this change intemperature could be directly used to control the setting of the oxygenwithdrawal valve, the purity of the oxygen could be directly controlled.This change in temperatureof the liquid oxygen in the "receiver is not,-however, available for directly actuating an'oxygen valve controlmechanism; first, because it is of very small magnitude, and second,becauseit is impossible to avoid some small fluctuations inpressurewhich 'vary' the boiling point of oxygen of constantcomposition.

The effect of changes in column pressure may drop and a concentration ofnitrogenisbeginning to appear.

In an air fractionating column, the tempera- .ture increases from plateto plate, downwardly;

and is at the maximum in the oxygen receiver. The temperature incrementsfrom plate to plate are small at the ends of the column where the changein composition from one plate to the next is slight, but at anintermediate point, par- .ticularly in the region of maximum argonconcentration where the nitrogen concentration is negligible, theincrement from plate to plate is relatively large.

At this level, spaced from the oxygen receiver by about one-third thetotal number of plates, the change in temperature following from a givenchange inpurityof the oxygen in the bottom of the column is from five toten times as great as the changein temperature in the receiver itself.This temperature differential becomes sufficient to control accuratelythe width of opening of the oxygen valve, through temperature responsivemeans located within the column at the two levels, i. e-., at thereceiver and in the neighborhood of a suitably selected level spacedfrom it where the concentration of the: argon is at a maximum but isjust beginning to decrease and the concentration of the nitrogen isnegligible but is. just beginning to increase.

Certainpractical applications of the invention will be described withreference to the attached drawings and the following descriptionsthereof,

in which:

Fig. 1 is a diagrammatical view of the medial form of the invention inwhich the differential expansions and contractions of two fluids arecaused to actuate the needle of an oxygen outlet valve throughmechanical linkage;

Fig. 3 is a diagrammatical view illustrating a form of the invention inwhich fluid expansion is used to actuate the valve needle through aninterposed solenoid;

Fig. 4 is a diagrammatical view illustrating a form of the invention inwhich the expansions of rigid metallic elements actuate the valve stemthrough mechanical linkage and,

Fig. 5 is a diagrammatical view illustrating a form of the inventionshowing a control device which does not depend on expansion to actuatethe control elements.

Referring first to Fig. 1, reference numeral I indicates the medialportion of a two-stage fractionating column of conventional design, thehigh pressure section I I being separated from the low pressure section,of which a fragment is indivcated at I2, by a partition plate l3 and anitrogen condenser I4. The stack of fractionating plates l separatesoxygenin a desired approach to purity. This liquid collects in a pool l6surrounding the condenserand is kept constantly are connected throughcapillary tubes I9 and 2t 4 boiling in condensing nitrogen vapor evolvedat considerably higher pressure in the high pressure section II.

A fluid-filled bulb IT is located at some convenient level in or closelyadjacent to oxygen pool l6, preferably a short distance below its normalsurface level. A. similar fluid-filled bulb I8 is placed on or near'aplate higher up in the column, preferably in the region near the maximumargon concentration as described above. These bulbs with. appropriateparts of a mechanism 2! illustrated in Fig. 2, which will next bedescribed. It will be understood that while the location of the lowerbulb. in or very close to the liquid oxygen pool is essential, the otherbulb may be located on any plate above it, the responsiveness of theapparatus increasing to a maximum at the plate where the argonconcentration is at a maximum but is just beginning to decrease and theconcentration of the nitrogen is negligible but is beginning toincrease,which is about one-third the way upthe-lengthof the low pressure column.7

Above this level, the. responsiveness diminshes. Thus, in thespecification and claims, the term is used in the neighborhood of thelevel at which the argon concentration is beginning to decrease from amaximum. and the nitrogen concentration is negligible butv isbeginningto increase. This term is meant to include a range of a numberof plates wherein the control apparatus will be operative. Whenoriginally designing a plant, the level for placing, the uppertemperature responsive means is 'calculated for the particular purity ofoxygen which. the plant is designed to produce.

If, however, in operation of the plant, thepurity ofthe. oxygen is to bevaried, the same upper level can be used to control the rate ofwithdrawal of the oxygen product; For purities other than those forwhich the plant was designed, the upper level will still be in theneighborhood of the point where the argon concentration is beginning todrop from a maximum and the nitrogen concentration is beginning toincrease from a negligible amount. Although the level will not be at thepreferred point, the apparatus will still operate satisfactorily andincorporate the advantages of the invention.

In Fig. 2, 22' is a valve body having an orifice 23- in which a taperingneedle 24' is reciprocated to vary the width of the annular passagethrough which oxygen fiows. This body is suitably mounted in oxygenwithdrawal pipe 25. Oxygen withdrawal pipe 25 may be located eitherbelow the liquid oxygen level or above it, for removal of the oxygen asa liquid or as a gas respectively.

A rig-id yoke 26 is firmly attached to the oxygen pipe .or valve body orotherwise accurately positioned with regard to the latter. Within'thisyoke are mounted two Sylphon bellows 21 and 28, opposing each other inaction. Projections 29 and 30- from the free ends of the bellows engageopposite sides of a lever 3| which is pivoted in one arm of the yoke asat 3,2. The free end of the lever. is linked at slot 33 to a valve stem34 to which valve needle 24 is attached. If this attachment is by athreaded connection as at 35 the needle may be adjusted manually to findthe valve opening which gives the required oxygen purity under normaloperating conditions. This is an initial adjustment only, not theoperative control. 7

The capillary tubes l9 and 20 are so connected to the respective bellowsthat the expansion of fluid in thelower bulb l1 tends to move the needle"downwardly or in a closing direction while the expansion of fluid inupper bulb l8 tends to move the needle upwardly and thus increase thevalve opening. Assuming the fluid in each bulb'to be a gas undercompression, an increase in the purity of the oxygen in pool l6 causesits temperature to rise and: thus" causes the pressure inbulb H andbellows-2T:to:-increase. Atthe sametimega much greater increase intemperature occurs in the liquid surrounding bulb l8 and the pressure inthis bulb and in bellows 28 increases to a greater degree. As thesepressures act on-opposite sides of lever 3|, the .force tendingtolif-tthe valve stem and increase the width-ofthe fiow opening is a functionof .the temperature sj difference between the two levels in the column.With suitable proportioning' of bulb "capacities, initial gas pressuresand linkages, all ofwhich'are subject to exact calculation in any giveninstance, the changing value of the temperature difference between the.two levels maybe. caused .tomove the valve stem and needleautomatically tojnew positions a at which the changed rate of. flowthrough the valve offsets the change in purity of the oxygen. and thuscauses it to return. to its original value.- y

When there is adecrease in the purity of the oxygen. in pool l6, itstemperature drops .and thus causes the pressure in bulb l1 and bellows21 to decrease. At the same time, a muchgreater, decrease in,temperature occurs in the liquid surrounding bulb l8. and the pressurein this bulb, and inbellows 28, decreases to a greater degree. As thesepressures act on opposite sides of lever 3|, the force tending to lowerthe valve stem and decrease the width of the flow opening is a functionof the increase in temperature difference between the two levels in thecolumn. This changing value of the temperature differencebetween the twolevels causes the .valve stem and needle to movedownwardly so that thenew position of the needle. decreases the rate of flow 4 and thus thepurityv of the product isincreased.

In thevuse of gasfilled bulbs, any gas which remains in the gaseous.phase at the lowest temperatures occurringinthis portion of the columnand at thepressuretowhich the bulbs are filled may be utilized, as forexample nitrogen, hydrogen orhelium. ...Or a liquid which does notfreeze at eolumntemperatureland which has areasonably low vapor.pressure at atmospheric tempera ture,. ,as forexample propane orpropylene, may be used. If the bulbs are charged with liquid, it isnecessary to substitute a resilient element, such as a stiff, open-coilspring, for each of the rigidpins .29 and 30.

A form of indirect actuation of the valve stem by differentialtemperature changes is illustrated in Fig. 3, in .whichseveral of theparts may be identical with similarly numbered parts in Fig. 2. In thisfor m, thevalve stem 34 is an extension of the core 36 of a solenoid 31and is urged downwardly by a spring 38 bearing on a collar 39, thesolenoid being spaced from the valve by a rigid yoke 40. In this form,changes in the pressures in the bellows 21 and 28 cause the arm 3!totravel across the contact points ofa'rheostat indicated at 4|, thusvarying the amperage of an electrical current and the extent to whichthe core, and with it the valve stem, .are retracted against "thedownward pressure exerted by the spring.

In the form illustrated in Fig. 4, the expansion and contraction ofmetallic rods is applied through mechanical linkage to raise and lower:Itwilhbeevident that the utilization of these 6 the valve stem. ,Inthis'figure, 22 indicates only the top of the stuffing box of the valveand l2-l2 are fragments of the shell of the column, separated toindicate a wide separation in the column length. Ayoke 42 of a metalhaving a relatively low coeflicient of expansion, such as irort ornickel, is fixedto the inner wall of the shellyand'arod 4.3 of a metalhaving a high coeificientotexpansion, such as copper, or aluminum,isattached to the inner end of the yoke and passed through the wall of theshell as through a smiling box or equivalent 44. A bracket 45 aflixed tothe outer wall of the shell carries a bell --crank;4 and a projectingarm 41, the latter bearing on the upper end of a spring 48 of which thelower end bears on a collar 49 fixed to valve 48A and; urging valve stem34 upwardly when rod 43A expands.

Iheaboye ,described structures have the common -iunctionsof producingexpansion with rising temperature ofexpansible elements-gaseous, liquidorlsolid+located respectively at that level inthe column at which theoxygen is at the maximymmomentary purity and at a higher level at which,thecolumnfiuids contain less oxygen; of utilizing the expansion andcontraction of the lower lyirig element to urge the oxygen-with-.draw-al-controlvalve in a closing or opening direction, respectively,and of utilizing the expansionand eontraction. of the higher-lyingelement .to urge-the withdrawal-control valve in an opening or closingdirection, respectively. This combination :ofsteps takes advantage ofthe greater degreeof temperature change at the higher level toadd-tothesensitivity of the control and neutralizes any effect ontemperature following from p fsssllrephanges of normal magnitude withinthe expansions'and contractions may be brought about inmany ways and bymany devices of which H113 above described" structures are suggestive la-3i;

Ingthepform. illustrated in Fig. 5, a control de- Y Ceis shown-whichdoesnot depend on expansionlofla' gas,-=liquid or solid to actuate thecontr'olelements. In-placeof the bulbs I! and I8 show'nsin. Fig. 1,resistance coils of a material whichachanges-tappreciably in resistivitywith change .in. temperature are-used. The resistancecoils.are-'.branched from a current source 52, 53 th'ro'ughlines l 9Aand ZOA-and return lines I93 and 2013:: Intrposed in lines Ian and 20Bare opposed minute solenoids 54,- 55 which actuate rods. 50, 51-," whichinturn actuate an arm 3| which.travels across-the contact points of arheostat -4ll fI hus, changes-in resistance of the resistance coilschangesthe amount of current flowing through minute solenoids 54, 55which in turn acting througharm -3l vary the amperage of an electrical=current-to control a large solenoid linl zeditotheva-lveas ln'Fig. 3.

I-claim: -1'. -Appa a llS fO1 stabilizing the purity of 'oxy gncollecting- -inthe lower end of an air fractionating column having anoxygen withdrawal line comprising, means sensitive to temperaturechanges at a level in the column where the oxygen is at the maximumpurity, means sensitive to temperature changes at another level in thecolumn in the neighborhood of the point where the argon concentration isbeginning to decrease from maximum concentration and the nitrogenconcentration is low but is beginning to increase,

and means actuated by the first and second claimed means in response tochanges in'the temperature differential between the, two'levels tocontrol the rate of the oxygen withdrawal throughthe withdrawal line. 7

2. Apparatus for stabilizing the purity of a liquid product collectingin the lower end of a fractionating column for a tertiary mixture ofdifierent boiling points which may include inconsequential proportionsof additional components, the column having a first component withdrawalvalve, comprising, temperature responsive means located in the column atsubstantially v the level of maximum momentary purity of the nectingeach of the pressure chambers with one of the temperature responsivemeans, the conduit connections and operative connections being soarranged that an increase in the temperature difierential between thetwo levels results in the actuation of the valve in aclosing directionwhereas a decrease in the temperature differential between the twolevels results in the actuation of the valve in an opening direction.

3. Apparatus for stabilizing the purity of the highest boilingpoint'product of a fractionating column separating mixtures having atleast three components of different boiling points, the col-- umn havinga highest boiling point product withdrawal line, comprising meanssensitive to temperature changes at one level in the column where thehighest boiling? point product is at the maximum purity, means sensitiveto temperature changes at a second level in thecolumn in theneighborhood where the concentration of a lower boiling point componentis beginning to decrease from'maximum and the concentration of a stilllower boiling point component is low but is beginning to increase; andmeans actuated by the first and second claimed means'in response to thechanges in the temperature differential between the two levels tocontrol the rate of highest boiling point product Withdrawal through thewithdrawal line.

4. Apparatus for stabilizing the purity of the highest boiling pointproduct of a fractionating column separating tertiary mixtures ofdifferent boiling point components, which mixture may includeinconsequential proportions of additionalcomponents, the column having ahighest boiling point product withdrawal line, commeans sensitive totemperature changes at a second level in the column in the neighborhoodwhere the concentration: of the intermediate boiling point tertiarycomponent is beginning to decrease from a maximum and the concentrationof the lowest boiling component is low but is beginning to increase, andmeans actuated by the first and second claimed means in response to thechanges in the temperature differential between the two levels tocontrol the rate of the highest boiling point product withdrawal throughthe Withdrawal line. 7

5. The method of controlling the rate of Withdrawal of the highestboiling point product collecting in a fractionating column in whichmixtures having at least three components of different boiling pointsare separated to stabilize the purity of the highest boiling pointproduct, comprising utilizing the changes in the temperaturedifierential between a level in the column where the highest boilingpoint product is at the maximum purity and a second level in the columnin the neighborhood where the concentration of a lower boiling componentis beginnin to decrease from maximum and the concentration of a stilllower boiling point component is low but is beginning to increase tocontrol the rate of withdrawal of the highest boiling point product.

6. Themethod of controlling the rate of withdrawal of the highestboiling .point product collecting in a fractionatin column in whichtertiary mixtures of different boiling points are separated, whichmixture may include inconsequential proportions of additionalcomponents, to stabilize the purity of the highest boiling pointproduct, comprising utilizing the changes in the temperaturedifferential between a level in the column where the highest boilingpoint product is at the maximum purity and a second level in the columnin the neighborhood where the, concentration of the intermediate boilingpoint tertiary component is beginning to decrease from a maximum and theconcentration of the lowest boiling point component is low but isbeginning to increase to control the rate of withdrawal of the highestboiling point product.

7. The method of controlling the rate of withdrawal of oxygenproduct'ofan air fractionating column to stabilize the purity of the oxygenproduct withdrawn from the column, comprising utilizing the changes inthe temperature differential between a level in the column where theoxygen product isat the maximum purity and a second level in the columnin the neighborhood where the argon concentration is beginning to dropfrom a maximum and the concentration of the nitrogen is low but isbeginning to increase to control the rate of withdrawal of the oxygenproduct.

' CLARENCE J. SCHILLING.

REFERENCES CITED 7 "The following references are of record in the fileof this patent:

UNITED STATES PATENTS Number Name Date 1,395,466 Barbet Nov. 1, 19211,492,063 Barbet Apr. 29, 1924 2,022,809 Kramer Dec. 3, 1935 2,316,056De Baufre Apr. 6, 1943 2,380,417 De Baufre July 31, 1945

1. APPARATUS FOR STABILIZING THE PURITY OF OXYGEN COLLECTING IN THELOWER END OF AN AIR FRACTIONATING COLUMN HAVING AN OXYGEN WITHDRAWALLINE COMPRISING, MEANS SENSITIVE TO TEMPERATURE CHANGES AT A LEVEL INTHE COLUMN WHERE THE OXYGEN IS AT THE MAXIMUM PURITY, MEANS SENSITIVE TOTEMPERATURE CHANGES AT ANOTHER LEVEL IN THE COLUMN IN THE NEIGHBORHOODOF THE POINT WHERE THE ARGON CONCENTRATION IS BEGINNING TO DECREASE FROMMAXIMUM CONCENTRATION AND THE NITROGEN CONCENTRATION IS LOW BUT ISBEGINNING TO INCREASE, AND MEANS ACTUATED BY THE FIRST AND SECONDCLAIMED MEANS IN RESPONSE TO CHANGES IN THE TEMPERATURE DIFFERENTIALBETWEEN THE TWO LEVELS TO CONTROL THE RATE OF THE OXYGEN WITHDRAWALTHROUGH THE WITHDRAWAL LINE.